Electromagnetic survey systems are being used increasingly to explore for oil and gas on land. However, at present, practical methods for exploring for oil and gas in the offshore environment are restricted to the measurement of the natural magnetic and gravitational fields at the earth's surface, of the reflection of seismic energy from subsurface structures, or the seepage of chemical substances from mineral deposits beneath the sea floor into the seawater or atmosphere. Although passive techniques such as natural-source magnetotellurics can provide useful information about the lower crust and upper mantle, electromagnetic sounding techniques employing an active source are better suited for surveying subterranean formations within five to ten kilometers beneath the seafloor. Because practical techniques for active electromagnetic sounding of earth formations beneath the seafloor have not hitherto been known, the electrical structures of continental margins and offshore basins remain largely unknown, despite the scientific and economic importance of these areas.
Magnetotelluric methods for exploring the electrical structure of such relatively shallow geologic formations beneath the sea have limited practicality due to a lack of signal at the high frequencies (above 0.1 Hz) needed to probe rocks disposed at shallow depths beneath the seafloor. Incident electromagnetic waves having such high frequencies are attenuated by the sea. In shallow water, where this attenuation is reduced, electromagnetic noise may render such methods impractical.
"Resistivity" methods using an active source of direct electric current, or very low frequency alternating current (having a frequency sufficiently low that induction affects are insignificant), have been proposed for determining the apparent resistivity of geologic formations beneath the sea. For example, U.S. Pat. No. 4,298,840 issued Nov. 3, 1981 to Bischoff et al discloses a method and apparatus for determining the apparent resistivity profile of the seafloor using a bottom-towed cable. The bottom-towed cable of Bischoff et al includes a pair of current supply electrodes, for supplying to the water bottom low frequency alternating electric current. The bottom-towed cable also includes several receiver electrodes. The potential differences between pairs of the receiver electrodes are measured. Bischoff et al discloses no electromagnetic sounding method employing a variable frequency source, fails to discuss the optimal absolute spacing between the source and the center point of each receiver electrode pair, and fails to discuss the manner in which the optimal source operating frequency may be selected. Nor does Bischoff et al disclose any method for determining the depth of a resistive layer buried beneath the seafloor. Nor does Bischoff et al teach or suggest towing the source and receiver electrodes above the seafloor.
U.S. Pat. No. 3,052,836 issued Sept. 4, 1962 to G. W. Postma discloses a method and apparatus for marine electrical prospecting which uses an active alternating current source having two electrodes and an electrical receiver circuit having two electrodes. Postma requires that at least one current source electrode and both receiver circuit electrodes be towed through the water within a few feet of the seafloor. Postma teaches that there will inherently be electromagnetic coupling directly between the source and receiver circuits which coupling is independent of the characteristics of the earth formation beneath the seafloor. To eliminate the effect of this direct coupling on the signal measured by the receiver circuit, Postma teaches imposing an adjustable transfer impedance between the source and receiver circuits. Postma does not discuss how the distance between the source and receiver circuits may be adjusted to reduce the direct electromagnetic coupling therebetween. Postma discloses no preferred source-receiver spacing nor optimal source operating frequency. Postma fails to disclose any method for determining the depth of a resistive layer buried beneath the sea floor.
U.S. Pat. No. 2,531,088 issued Nov. 21, 1950 to R. R. Thompson discloses another method for measuring the resistivity of a geological formation beneath a body of water which involves dragging a cable, including a pair of current electrodes and a plurality of potential electrodes, along the bottom of the water. The source electrodes emit a very low frequency (less than 1/3 Hz) sinusoidal signal or a periodically reversed (at a frequency less than 2/3 Hz) DC signal. The potential difference between pairs of the potential electrodes are measured. Thompson fails to disclose any method using a variable frequency source or any method for determining the depth of a resistive layer buried beneath the water bottom.
U.S. Pat. Nos. 3,182,250, issued May 4, 1965 to F. M. Mayes; 2,872,638 issued Feb. 3, 1959 to S. B. Jones; and 2,839,721 issued June 17, 1958 to L. DeWitte also disclose methods for measuring the resistivity of subfloor earth formations employing DC or very low-frequency active sources, but do not disclose any method using a variable frequency source.
In addition to such resistivity methods, electromagnetic sounding methods using a variable frequency active source have been proposed. For example, the article "Electromagnetic Investigation of the Seafloor", Geophysics, Volume 35, No. 3 (June 1970), pages 476-489, by J. H. Coggon et al provides background discussion of the theory underlying such variable frequency active source methods, and suggests that the subfloor structure could be effectively probed by a system employing a vertical magnetic dipole source disposed at, or just above the seafloor and operated in a selected frequency range dependent on the conductivity of the subfloor structure and the source-to-receiver spacing. The Coggon et al paper discusses only a survey system employing a vertical magnetic dipole source and does not suggest any method using a tuned receiver array or any method for determining the depth of a buried resistive layer.
U.S. Pat. No. 4,047,098 issued Sept. 6, 1977 to Duroux discloses an electromagnetic active source sounding method employing an electric or magnetic dipole source and receivers towed at the surface of a body of water behind a seismic vessel. Duroux employs a pair of electrodes as a receiver for measuring the component of the electric field transverse to the direction of tow. Duroux also suggests measuring the radial and vertical components of the magnetic field at the surface of the water but does not describe any particular apparatus for so measuring the radial and vertical magnetic fields. Duroux teaches that the source-receiver spacing should be large relative to the desired investigation depth, and should preferably be at least double the desired investigation depth. Duroux does not suggest a preferred operating frequency for the dipole source and does not suggest disposing the source or receiver near the seafloor or indeed at any position below the surface of the body of water. Nor does Duroux suggest any method using a tuned array of electric dipole receivers or any method for determining the depth beneath the seafloor of a buried resistive layer.